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Poly(methylmethacrylate) (PMMA) bone cement, used to fix implants into the bone, produces good surgical results if used correctly. However, prostheses do eventually become loose and the breakdown of the cement mantle is a factor in this failure. Limitations of PMMA cement, which lead to problems with the fixation of the implant, include its mechanical characteristics and its influence upon surrounding bone, associated with the polymerization reaction. A bioactive bone cement is particularly designed to produce a better interface between the cement and bone. However, an improvement in mechanical properties, especially fatigue, creep and fracture toughness, are an added necessary requirement to increase the lifetime of a cemented implant. The development of a bioactive cement has been conducted mainly in two ways; firstly, to improve existing PMMA cement by the addition of various bioactive agents and secondly, to design an alternative matrix for the bioactive material to be combined with. The most promising investigations which have been conducted, along with their relative benefits and drawbacks, are discussed.

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The mechanical properties of acrylic bone cement, used in orthopedic surgery, are very influential in determining successful long-term stability of a prosthesis. A large number of commercial formulations are available, differing in chemical composition and physical properties of both powder and monomer constituents. In this study, the static and dynamic tensile characteristics of a number of the most commonly used bone cements (Palacos R, Simplex P, CMW 1 & 3, Sulfix-60, Zimmer Dough), along with some newer formulations (Endurance, Duracem 3, Osteobondtrade mark and Boneloc), have been investigated under the same testing regimes. Testing was performed in air at room temperature. Significant differences in both static and fatigue properties were found between the various bone cements. Tensile tests revealed that Palacos R, Sulfix-60, and Simplex P had the highest values of ultimate tensile strength, closely followed by CMW 3, while Zimmer Dough cement had the lowest strength. Fatigue testing was performed under stress control, using sinusoidal loading in tension-tension, with an upper stress level of 22MPa. The two outstanding cements when tested in these cyclic conditions were Simplex P and Palacos R, with the highest values of Weibull median cycles to failure. Boneloc bone cement demonstrated the lowest cycles to failure. While the testing regimes were not designed to replicate exact conditions experienced by the bone cement mantle in vivo, there was a correlation between these results and clinical outcome.

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The nature of the orthopedic implant surface affects the interaction between cells and subsequent bone formation. The bone/cement interface in cement-held prostheses is considered to be the main cause of fracture leading to implant revision. It is thought that the introduction of a bioactive phase, such as hydroxyapatite (HA), to cement may permit a stronger implant by encouraging direct bone apposition rather than encapsulation of the implant by fibrous tissue. Thus, a poly(methylmethacrylate) (PMMA) cement incorporating 17.5% HA by weight has been investigated. In this study, in order to analyze the interaction at the cellular level, the in vitro biological response of the HA/PMMA to a similar PMMA without HA incorporation has been studied. Primary human osteoblast-like cells (HOB) were used as they are a model of the cell type the cements might encounter in vivo. Cell proliferation and growth were assessed by measurement of total cellular DNA and tritiated thymidine ([3H]-TdR) incorporation. Alkaline phosphatase (ALP) production was measured as an indicator of HOB phenotype upon the cements. The results showed that HA/PMMA was a better substrate for HOB cells, resulting in increased proliferation and ALP activity. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) showed that HOB cells cultured on the HA-filled PMMA preferentially anchored to HA particles exposed at the cement surface, with a close intimacy observed between HA and HOB cells.